X-ray fiber diffraction study on industrially important polysaccharides
Abstract
The three-dimensional structures of four polysaccharides, RMDP17, beijeran, guaran, and alginic acid, have been determined by x-ray fiber diffraction analysis. RMDP17, a rhamsan analog, adopts a gellan-like double helix, with its side units folded toward and hydrogen-bonded to the main chain. Two helices of opposite polarity occupy a trigonal unit cell. In sodium beijeran, two extended, 2-fold, antiparallel helices are nestled tightly in a monoclinic unit cell, leading to a thick-sheet structure in the crystalline lattice with ions and water embedded between sheets. The study of guaran was to rectify a previous morphology report, in which the galactosyl side unit had been fixed in the β-D instead of the correct α-D configuration. The correct results show that guaran forms a flat, 2-fold helix, stabilized by periodic intramolecular hydrogen bonds between the galactosyl side units and the mannan main chain. Ordered water molecules are responsible for formation and association of sheets of guaran helices in the crystal lattice. The structure of alginic acid was re-examined using published x-ray diffraction data from poly(mannuronic acid) and poly(guluronic acid). In both structures, the carboxylate group orientations appear in quite different domains from those in the preliminary model. In the solid state, polymannuronic acid helices associate to form flat, thin sheets and adjacent sheets are closely packed so that there is no place for guest molecules in the structure, perhaps related to its inability to gel; whereas in poly(guluronic acid), water columns are found to connect four surrounding polymer chains. This morphology could be extended to explain the gelling behavior of alginate in the presence of calcium ions.
Degree
Ph.D.
Advisors
Chandrasekaran, Purdue University.
Subject Area
Food science|Agricultural engineering
Off-Campus Purdue Users:
To access this dissertation, please log in to our
proxy server.